Method of manufacturing an optical system

ABSTRACT

A method of manufacturing a projection optical system ( 37 ) for projecting a pattern from a reticle to a photosensitive substrate, comprising a surface-shape-measuring step wherein the shape of an optical test surface ( 38 ) of an optical element ( 36 ) which is a component in the projection optical system is measured by causing interference between light from the optical surface ( 38 ) and light from an aspheric reference surface ( 70 ) while the optical test surface ( 38 ) and said reference surface ( 70 ) are held in integral fashion in close mutual proximity. A wavefront-aberration-measuring step is included, wherein the optical element is assembled in the projection optical system and the wavefront aberration of the projection optical system is measured. A surface correction calculation step is also included wherein the amount by which the shape of the optical test surface should be corrected is calculated based on wavefront aberration data obtained at the wavefront-aberration-measuring step and surface shape data obtained from the surface-shape-measuring step. The method also includes a surface shape correction step wherein the shape of the optical test surface is corrected based on calculation performed at the surface correction calculation step. Surface shape measuring interferometer systems and wavefront-aberration-measuring interferometer systems ( 22 J- 22 Q) used in performing the manufacturing method are also disclosed.

FIELD OF THE INVENTION

The present invention relates to an interferometer system for measuringthe shape of an aspheric surface of an optical element in an opticalsystem and for measuring the wavefront aberration of such an opticalsystem, particularly in connection with manufacture of a projectionoptical system suited to for use in an exposure apparatus employing softX-ray (EUV) exposure light.

BACKGROUND OF THE INVENTION

Light of wavelength 193 nm or longer has hitherto been used as theexposure light in lithographic equipment used when manufacturingsemiconductor devices such as integrated circuits, liquid crystaldisplays, and thin film magnetic heads. The surfaces of lenses used inprojection optical systems of such lithographic equipment are normallyspherical, and the accuracy in the lens shape is 1 to 2 nm RMS (rootmean square).

With the advance in microminiaturization of the patterns onsemiconductor devices in recent years, there has been a demand forexposure apparatus that use wavelengths shorter than those usedheretofore to achieve even greater microminiaturization. In particular,there has been a demand for the development and manufacture ofprojection exposure apparatus that use soft X-rays of wavelength of 11to 13 nm.

Lenses (i.e., dioptric optical elements) cannot be used in the EUVwavelength region due to absorption, so catoptric projection opticalsystems (i.e., systems comprising only reflective surfaces) areemployed. In addition, since a reflectance of only about 70% can beexpected from reflective surfaces in the soft X-ray wavelength region,only three to six reflective surfaces can be used in a practicalprojection optical system.

Accordingly, to make an EUV projection optical system aberration-freewith just a few reflective surfaces, all reflective surfaces are madeaspheric. Furthermore, in the case of a projection optical system havingfour reflective surfaces, a reflective surface shape accuracy of 0.23 nmRMS is required. One method of forming an aspheric surface shape withthis accuracy is to measure the actual surface shape using aninterferometer and to use a corrective grinding machine to grind thesurface to the desired shape.

In a conventional surface-shape-measuring interferometer, measurementrepeatability is accurate to 0.3 nm RMS, the absolute accuracy for aspherical surface is 1 nm RMS, and the absolute accuracy of an asphericsurface is approximately 10 nm RMS. Therefore, the required accuracycannot possibly be satisfied. As a result, a projection optical systemdesigned to have a desired performance cannot be manufactured.

So-called null interferometric measurement using a null (compensating)element has hitherto been conducted for the measurement of asphericsurface shapes. Null lenses that use spherical lenses comprisingspherical surfaces, and zone plates wherein annular diffraction gratingsare formed on plane plates have principally been used as null elements.

FIG. 1 shows a conventional interferometer system 122 arrangement fornull measurement using a null (compensation) element 132. Theinterferometric measurement described herein is a slightly modifiedversion of a Fizeau interferometric measurement. Namely, a plane wave126 emitted from an interferometric light source 124 is partiallyreflected by a high-precision Fizeau surface 130 formed on a Fizeauplane plate 128. The component of plane wave 126 transmitted throughFizeau surface 130 is converted into measurement wavefront (nullwavefront) 134 by null element 132 and assumes a desired aspheric designshape at a measurement reference position RP, following which it arrivesat a test surface 138 of a test object 136 previously set at thereference position. The light arriving at test surface 138 is reflectedtherefrom and interferes with the light component reflected from Fizeausurface 130, and forms monochromatic interference fringes insideinterferometer system 122. These interference fringes are detected by adetector such as a CCD (not shown). A signal outputted by the detectoris analyzed by an information processing system (not shown) thatprocesses the interferometer information contained in the output signal.Similar measurements can be performed using a Twyman-Greeninterferometer. To accurately ascertain the shape of test surface 138,the null element 132 must be manufactured with advanced technology,since there must be no error in the null wavefront. Specifically, thismeans that the optical characteristics of the null element 132 must bemeasured beforehand with high precision. Based on these measurements,the shape of null wavefront 134 is then determined by ray tracing. Thisresults in the manufacture of null element 132 taking a long time.Consequently, the measurement of the desired aspheric surface takes along time.

FIG. 2 shows another example of a conventional Fizeau interferometer222. Referring to FIG. 2, laser light from laser 224 passes through alens system 226 to become a collimated light beam of a prescribeddiameter and is incident Fizeau plate 228. Rear side 230 of Fizeau plate228 is accurately ground to a highly flat surface, and the component ofthe incident light reflected by rear side 230 of Fizeau plate 228becomes a reference beam having a plane wavefront. The component ofincident light transmitted through a Fizeau plate 228 passes throughnull element 232, where the plane wavefront where the plane wavefront isconverted to a desired aspheric wavefront. The aspheric wavefront isthen incident in perpendicular fashion an aspheric test surface 238. Thelight reflected by test surface 238 returns along the original opticalpath, is superimposed on the reference light beam, reflects off a beamsplitting element 256 in lens system 226, and forms interference fringeson a CCD detector 260. By processing these interference fringes by acomputer (not shown), the shape error can be measured.

A problem with interferometer 222 is deterioration, in absoluteaccuracy, due to null element 232. A null element comprising a number ofhigh-precision lenses (e.g., lenses 234 and 236) a CGH(computer-generated hologram), or the like is ordinarily used as nullelement 232, and manufacturing errors on the order of 10 nm RMStypically result.

Since interferometer 222 tends to be affected by vibration and airfluctuations due to the separation of reference surface 230 (i.e., rearside of Fizeau plate 228) and test surface 238. Repeatability is alsopoor, at 0.3 nm RMS. Furthermore, in measuring an aspheric surface,alignment of null element 232 and test surface 238 is critical.Measurement repeatability deteriorates by several nanometers ifalignment accuracy is poor.

SUMMARY OF THE INVENTION

The present invention relates to an interferometer system for measuringthe shape of an aspheric surface of an optical element in an opticalsystem and for measuring the wavefront aberration of such an opticalsystem, particularly in connection with manufacture of a projectionoptical system suited to for use in an exposure apparatus employing softX-ray (EUV) exposure light.

The goal of the present invention is to overcome the above-describeddeficiencies in the prior art so as to permit fast and accuratecalibration of a null wavefront corresponding to an aspheric surfaceaccurate to very high dimensional tolerances.

Another goal of the present invention is to manufacture a projectionoptical system having excellent performance.

Additional goals of the present invention are to provide anaspheric-surface-shape measuring interferometer having goodreproducibility, to measure wavefront aberration with high precision andto permit calibration of an aspheric-surface-shape measuringinterferometer so as to improve absolute accuracy in precision surfacemeasurements.

Accordingly, a first aspect of the invention is an interferometercapable of measuring a surface shape of a target surface as compared toa reflector standard. The interferometer comprises a light sourcecapable of generating a light beam, and a reference surface arrangeddownstream of the light source for reflecting the light beam so as toform a reference wavefront. The interferometer further includes a nullelement arranged downstream of the reference surface for forming adesired null wavefront from the light beam. The null element is arrangedsuch that the null wavefront is incident the target surface so as toform a measurement wavefront and is also incident the reflector standardwhen the latter is alternately arranged in place of the target surfaceso as to form a reflector standard wavefront. The interferometer furtherincludes a detector arranged so as to detect interference fringes causedby interference between the measurement wavefront and the referencewavefront. The detection of the interference fringes takes into accountthe reflector standard wavefront.

A second aspect of the invention is a method of manufacturing aprojection optical system capable of projecting a pattern from a reticleonto a photosensitive substrate. The method comprises the steps of firstmeasuring a shape of a test surface of an optical element that is acomponent of the projection optical system by causing interferencebetween light from the test surface and light from an aspheric referencesurface while the test surface and the aspheric reference surface areheld integrally and in close proximity to one another. The next step isassembling the optical element in the projection optical system andmeasuring the wavefront aberration of the projection optical system. Thenext step is then determining an amount by which the shape of the testsurface should be corrected based on the measured wavefront aberrationobtained in the step b. Then, the final step is correcting the shape ofthe test surface based on the amount by which the shape of the testsurface should be corrected as determined above.

A third aspect of the invention is an interferometer for measuringwavefront aberration of an optical system having an object plane and animage plane. The interferometer comprises a light source for supplyinglight of a predetermined wavelength, a first pinhole member capable offorming a first spherical wavefront from the light arranged at one ofthe object plane and the image plane. The first pinhole member has aplurality of first pinholes arrayed in two dimensions along a surfaceperpendicular to an optical axis of the optical system. Theinterferometer further includes a second pinhole member arranged at theopposite one of the object plane and the image plane of the firstpinhole member. The second pinhole member has a plurality of secondpinholes arrayed at a position corresponding to the imaging positionwhere the plurality of first pinholes is imaged by the optical system.The interferometer also includes a diffraction grating arranged in theoptical path between the first and second pinhole members, and adiffracted light plate member that selectively transmits diffractedlight of one or more higher predetermined diffraction orders associatedwith the diffraction grating. The interferometer also includes adetector arranged to detect interference fringes arising from theinterference between a second spherical wavefront generated by a zeroethdiffraction order passing through the second pinhole member and the oneor more higher predetermined diffraction orders passing through thediffracted light plate member.

A fourth aspect of the invention is an interferometer calibration methodfor measuring a surface shape of an optical element of an opticalsystem. The method comprises the steps of first, interferometricallymeasuring the surface shape of the optical element to obtain a surfaceshape measurement value, then assembling the optical system by includingthe optical element in the optical system, then measuring a wavefrontaberration of the optical system, then separating the wavefrontaberration into a component corresponding to positional error of thesurface shape and a component corresponding to surface shape error, thencorrecting the positional error component and calculating the surfaceshape error component then finally correcting the surface shapemeasurement value using the surface error component as previouslycalculated.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic optical diagram of a first conventionalsurface-shape-measuring interferometer according to the prior art;

FIG. 2 is a schematic optical diagram of a second conventionalsurface-shape-measuring interferometer according to the prior art;

FIGS. 3a and 3 b are schematic optical diagrams of first and secondsurface-shape-measuring interferometers of a first embodiment accordingto a first aspect of the present invention;

FIGS. 4a and 4 b are schematic optical diagrams of third and fourthsurface-shape-measuring interferometers of a first embodiment accordingto a first aspect of the present invention;

FIGS. 5a and 5 b are schematic optical diagrams of fifth and sixthsurface-shape-measuring interferometers of a second embodiment accordingto a first aspect of the present invention;

FIG. 6 is a schematic optical diagram of a seventhsurface-shape-measuring interferometer of a third embodiment accordingto a first aspect of the present invention;

FIG. 7 is a schematic optical diagram of an eighthsurface-shape-measuring interferometer of a fourth embodiment accordingto a second aspect of the present invention;

FIGS. 8a and 8 b are cross-sectional diagrams of the main components ofthe holder assembly of the surface-shape-measuring interferometer ofFIG. 7;

FIG. 9 is a schematic optical diagram of a ninth surface-shape-measuringinterferometer that is a variation of the surface-shape-measuringinterferometer of FIG. 7;

FIG. 10a is a schematic optical diagram of a firstwavefront-aberration-measuring interferometer for explaining theprinciple of a fifth embodiment according to a third aspect of thepresent invention;

FIG. 10b is a cross-sectional diagram of a second semitransparent filmwith a pinhole plate in the interferometer of FIG. 10a;

FIG. 11a is a schematic optical diagram of a secondwavefront-aberration-measuring interferometer that is a variation of thewavefront-aberration-measuring interferometer of FIG. 10a;

FIG. 11b is a plan view of the second dual hole plate in theinterferometer of FIG. 11a;

FIG. 11c is a cross-sectional diagram explaining the operation of thesecond dual hole plate in the interferometer of FIGS. 11a and 11 b;

FIG. 12 is a schematic optical diagram of a thirdwavefront-aberration-measuring interferometer of a fifth embodimentaccording to a third aspect of the present invention;

FIG. 13a is a plan view of a first embodiment of the first pinhole arrayplate of the interferometer of FIG. 12;

FIG. 13b is a plan view of a first embodiment of the second dual holearray plate of the interferometer of FIG. 12;

FIG. 14a is a plan view of a second embodiment of the first pinholearray plate, being a variation on the first embodiment of the firstpinhole array plate of FIG. 13a;

FIG. 14b is a plan view of a second embodiment of the second dual holearray plate, being a variation on the first embodiment of the seconddual hole array plate of FIG. 13b;

FIG. 15a is a schematic optical diagram of fourthwavefront-aberration-measuring apparatus of a sixth embodiment accordingto the present invention;

FIG. 15b is a plan view of second Hartmann plate of the apparatus shownin FIG. 15a;

FIG. 16a is a schematic optical diagram of a fifthwavefront-aberration-measuring interferometer of a seventh embodimentaccording to a third aspect of the present invention;

FIG. 16b is a plan view of the first pinhole cluster plate of the ininterferometer of FIG. 16a;

FIG. 16c is a plan view of the second dual hole cluster plate of the ininterferometer of FIG. 16a;

FIG. 17a is a plan view of the first pinhole row plate of an eighthembodiment according to a third aspect of the present invention;

FIG. 17b is a plan view of the second dual hole row plate in an eighthembodiment according to a third aspect of the present invention;

FIG. 18a is a plan view of the first slit plate of a ninth embodimentaccording to a third aspect of the present invention;

FIG. 18b is a plan view of the second dual slit plate of a ninthembodiment according to a third aspect of the present invention;

FIG. 19 is a schematic optical diagram of a sixthwavefront-aberration-measuring interferometer of a tenth embodimentaccording to a third aspect of the present invention;

FIG. 20a is a schematic optical diagram of seventhwavefront-aberration-measuring interferometer of an eleventh embodimentaccording to a third aspect of the present invention;

FIG. 20b is a cross-sectional diagram of the second pinhole mirror platein the interferometer of FIG. 20a;

FIG. 21a is a plan view of the first pinhole array plate used in avariation of the interferometer of FIG. 20a;

FIG. 21b is a plan view of second pinhole mirror array plate 63 in avariation on interferometer 22Q shown in FIG. 20a;

FIG. 22 is a schematic optical diagram of an eighthwavefront-aberration-measuring interferometer that is a variation of theinterferometer of FIG. 20a;

FIG. 23 is a schematic optical diagram of awavefront-aberration-measuring apparatus serving as a comparativeexample for illustrating the advantage of interferometers of FIGS. 20aand 22;

FIG. 24 is a flowchart indicating an exemplary method for calibratingthe aspheric-surface-shape measuring interferometer of FIG. 7 using thewavefront-aberration-measuring interferometer FIG. 10a; and

FIG. 25 is a cross-sectional showing a small tool grinding apparatusused in the interferometer calibration method indicated in the flowchartof FIG. 24.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to an interferometer system for measuringthe shape of an aspheric surface of an optical element in an opticalsystem and for measuring the wavefront aberration of such an opticalsystem, particularly in connection with manufacture of a projectionoptical system suited to for use in an exposure apparatus employing softX-ray (EUV) exposure light.

Referring to FIGS. 3a and 3 b, the principle of operation of aninterferometer system according to a first aspect of the presentinvention is now discussed.

Compared with prior art interferometer 122 shown in FIG. 1, first andsecond interferometer systems 22A and 22B, shown in FIGS. 3a and 3 b,respectively, according to the first aspect of the present inventionhave a reflective standard 40 with a separately and accuratelycalibrated spherical reflective surface 42 arranged in place of testsurface (aspheric surface) 138 of test object 136 (see FIG. 1).

Interferometer 22A shown in FIG. 3a further differs from prior artinterferometer 122 of FIG. 1 in that a wavefront 45 incident nullelement 32 is a spherical wavefront from a Fizeau lens 44, and in that aFizeau surface 46 is used as the reference surface. Fizeau lens 42 neednot be limited to a convergent system as shown, but may also be adivergent system.

Interferometer 22B shown in FIG. 3b is an example wherein a wavefrontincident null element 32 is a plane wave 26, as in the case of prior artinterferometer 122 shown in FIG. 1. A flat Fizeau surface 30 of a Fizeaulens 28 is used as the reference surface. Interferometer 22B differsfrom prior art interferometer 122 of FIG. 1 in that the light beamconverted by null element 32 is a convergent light beam, and in that itpermits measurement of concave surfaces as well as convex surfaces. Amethod of calibrating null wavefront in this case is to use a concavereflective surface to calibrate the wavefront as it diverges afterhaving first converged, and then to reverse calculate the shape of thenull wavefront 34 at the position where it is actually used (heavy linein drawing) based on the calibrated wavefront shape. High-precisioncalibration is possible if a pinhole interferometer (i.e., a pointdiffraction interferometer, hereinafter referred to as a “PDI,”discussed further below) is used to calibrate the concave reflectivesurface.

If the amount of asphericity of surface 42 is small, then the entiresurface can be measured all at once. However, in the case of an asphericsurface that unfortunately generates interference fringes exceeding theresolution of the interferometer CCD, data for the entire surface can beobtained in the same manner by applying the so-called wavefrontsynthesis technique. This technique involves axially displacingreflective standard 40 relative to null wavefront 34, conductinginterferometric measurements on a plurality of annular wavefront data,and joining the redundant regions of each of the data so they overlapwithout excess.

First Embodiment

Referring now to FIGS. 4a and 4 b, third and fourthsurface-shape-measuring interferometers 22C and 22D of a firstembodiment according to a first aspect of the present invention are nowdescribed, wherein a PDI 52 employing an ideal spherical wavefront froma point light source 54 is used to measure null element 32 in Fizeau(aspheric-surface-measuring) interferometer (i.e., first interferometersystem) 22A shown in FIG. 3a.

Interferometer 22C shown in FIG. 4a employs a divergent null element 32,and interferometer 22D shown in FIG. 4b employs a convergent nullelement 32. The latter is adopted when calibrating the wavefront 34 formeasurement of a convex surface.

Since spherical wavefront 45 incident null element 32 in interferometers22C and 22D of FIGS. 4a and 4 b is an ideal spherical wavefront from apoint light source 54, it is possible to simultaneously ascertain theshape of null wavefront 34 as well as the transmission characteristicsof null element 32.

Second Embodiment

Referring now to FIGS. 5a and 5 b, fifth and sixthsurface-shape-measuring interferometers 22E and 22F of a secondembodiment according to a first aspect of the present invention are usedto measure null element 32 generating a convergent null wavefront 34, asthe case at interferometer 22B shown in FIG. 3b. Interferometer 22E ofFIG. 5a uses a spherical wavefront 45 as the wavefront from Fizeausurface 46 incident null element 32. Interferometer 22F in FIG. 5b usesa plane wave 26 therefor. It does not matter whether spherical wavefront45 is a convergent light beam or a divergent light beam. Furthermore,use of PDI 52 replaces calibration using a reflective surface. PDI 52corresponds to a point light source of the present invention.

To perform measurements with PDI 52, taking the case in which nullwavefront 34 is convergent, pinhole 54 of PDI 52 is positioned so as toapproximately coincide with the point of convergence of null wavefront34. As a result, null wavefront 34, which is reflected from a reflectivesurface 56 surrounding pinhole 54, and the ideal spherical wavefrontproduced by the light leaving pinhole 54 will form interference fringes.

Third Embodiment

Referring now to FIG. 6, a seventh surface-shape-measuringinterferometer 22G is a third embodiment according to a first aspect ofthe present invention and is similar to interferometer 22E of FIG. 5a,except that a PDIs 52A is used in place of a Fizeau lens 44 that therehad generated a spherical wavefront. A second PDI 52B is also used formeasurement light. In interferometer 22E and 22F shown in FIGS. 5a and 5b, respectively, there is a possibility that during operation of Fizeauinterferometer 22E or 22F, the measurement light signal from PDI 52 willbe lost in noise. In this case, it is preferable to in addition employ apolarizing element to reduce noise and improve the usable signal.

The measurement arrangement in interferometer 22G shown in FIG. 6 hasthe advantage that pinhole 54B that forms the point light source ofsecond PDI 52B acts to reduce noise and improve the usable signal. Thispermits not only the shape of null wavefront 34 and the transmissioncharacteristics of null element 32 to be accurately calibrated, but alsopermits the transmission characteristics of two PDIs 52A and 52B to becalibrated in both the forward and backward directions. Accordingly,accuracy can be further improved.

To actually use one of the aforementioned interferometers 22C-22G tomeasure a test surface 38 after calibration has thus been performed,reflective standard 40, point light source forming means, PDIs 52 or thelike are removed and these are replaced with the original test surface38 and a light source 48, following which measurements may be performed.

As described above, interferometers 22C-22G of the first through thirdembodiments according to a first aspect of the present invention make itpossible to calibrate an aspheric null element 32 with high precisionand in a short period of time.

Fourth Embodiment

FIG. 7 shows an eighth surface-shape-measuring interferometer 22H of afourth embodiment according to a second aspect of the present invention.FIGS. 8a and 8 b show the principal parts of interferometer 22H of FIG.7. Interferometer 22H shown in FIG. 7 is capable of measuring the shapeof an aspheric surface.

Referring to FIG. 7, laser light from a laser 24 is changed into acollimated beam of a prescribed diameter by way of a lens system 58, andis then incident null element 32. Null element 32 emits a wavefronthaving a shape substantially identical to that of test surface 38, andthe wavefront, having been converted to a prescribed aspheric surfaceshape, is incident in perpendicular fashion, an aspheric referencesurface 70 and aspheric test surface 38. Furthermore, aspheric referencesurface 70 has substantially the same shape as aspheric test surface 38(with, however, concavity and convexity reversed). The light incidentaspheric reference surface 70 is amplitude-divided, with one wavefrontproceeding to test surface 38 and the other wavefront returning alongthe original optical path to serve as reference wavefront.

Aspheric reference surface 70 is arranged proximate test surface 38, andaspheric reference surface 70 and test surface 38 have mutuallycomplementary shapes. Aspheric reference surface 70 and test surface 38are supported in integral fashion by a holder 72.

Furthermore, light from aspheric reference surface 70 is reflected bytest surface 38, and is again incident aspheric reference surface 70 asthe measurement wavefront.

After the abovementioned reference wavefront and measurement wavefrontexit from the reference optical element 76 upon which aspheric referencesurface 70 is formed, they are incident null element 32, are reflectedby a beam splitter 74 within lens system 58, and then form interferencefringes on detector 60 comprising a CCD or other such image pickupelement. By processing these interference fringes with a computer CUelectronically connected to detector 60, the shape error of test surface38 can be measured.

In interferometer 22H shown in FIG. 7, a main body, which includes theelements from laser 24 to null element 32, and holder 72, are supportedby separate members so as to be spatially separated.

Interferometer 22H shown in FIG. 7 is basically a Fizeau interferometer,but it has several significant advantages over prior art Fizeauinterferometer 222 of FIG. 2. The causes of the degradation in themeasurement reproducibility in a conventional interferometer such asinterferometer 122 of FIG. 1 or interferometer 222 of FIG. 2 include airfluctuations, vibration, sound, air pressure fluctuations, temperaturefluctuations, detector noise, nonlinear errors and amplitude errors inthe fringe scan, reproducibility of positioning the specimen,reproducibility of strain in the specimen due to the specimen holder,and aberrations in the optical system. Among these, air fluctuations,vibration, sound, air pressure fluctuations, temperature fluctuations,and optical system aberrations can be significantly reduced by bringingtest surface 38 and reference surface 70 close together and physicallyjoining them in integral fashion, as in interferometer 22H of the fourthembodiment of the present invention shown in FIG. 7.

Particularly with respect to interferometer 22H of in FIG. 7, while nullelement 32 is used therein, measurement accuracy is not affected byeither the accuracy of null element 32 or the accuracy of alignmentbetween null element 32 and test surface 38. This is because nullelement 32 functions to deliver a wavefront having an aspheric shapemore or less identical to aspheric reference surface 70 to that asphericreference surface 70, but does not directly function to deliver anaspherically shaped wavefront to test surface 38. Accordingly, althoughnull element 32 is not an essential component in interferometer 22H, itis preferable to use null element 32 so as to improve measurementaccuracy.

The positional reproducibility of test object 36 in interferometer 22His ensured through use of a position sensor PS (electronic micrometer orthe like), not shown, arranged around test object 36, and thereproducibility of strain in the test specimen 36 from the specimenholder 72 is improved by constructing the specimen holder 72 such thatsupport is effected in three-point or multi-point fashion.

In addition, the close proximity of test surface 38 and referencesurface 70 makes detection of alignment error easier and enableshigh-precision alignment. Detector noise can be sufficiently reduced bycooling detector 60 and by integrating the data. Nonlinear errors andamplitude errors during fringe scans can be eliminated by using adigital-readout piezoelectric element, and by processing the signal suchthat there are an increased number of packets during fringe scans.Adoption of the above-described constitution in interferometer 22Hpermits attainment of repeatabilities of 0.05 nm RMS or better, andpermits attainment of measurement reproducibilities, including alignmenterror, changes occurring over time, and so forth, of 0.1 nm RMS orbetter.

A remaining problem with interferometer 22H is absolute accuracy, whichis dependant on the surface accuracy of reference aspheric surface 70.This error is a systematic error associated with the interferometer 22H.Below are described ways to correct this error (i.e., how calibration tooffset this error.

Interferometer 22H, while based on conventional Fizeau interferometer222 shown in FIG. 2, is different from the conventional Fizeauinterferometer in the following respects. Fizeau (reference) surface 70of interferometer 22H is an aspheric surface, its shape being such thatconvexity and concavity are reversed with respect to test surface 38arranged in close proximity to Fizeau surface 70. The constitution issuch that reference element 76 is separated from the optical system, andsuch that the (Fizeau) reference optical element 76 is physicallyconnected in integral fashion to test object 36. This constitutionsignificantly improves repeatability and measurement reproducibility ascompared with that of above-described conventional interferometer 222shown in FIG. 2.

FIGS. 8a and 8 b show two exemplary configurations for holder assembly72 of interferometer 22H of FIG. 7. FIG. 8a shows an exemplaryconfiguration wherein the spacing between test surface 38 and asphericreference surface 70 is variable, and FIG. 8b shows an exemplaryconfiguration wherein the spacing is fixed.

Referring to FIG. 8a, reference element 76 with aspheric referencesurface 70 is held by reference element holder 72H, which is disposedseparately from the interferometer 22H main body. A piezoelectricelement 72P is provided on reference element holder 72H. A test objectholder 72T, which holds test object 36, is mounted to reference elementholder 72H by way of piezoelectric element 72P. By driving piezoelectricelement 72P, the spacing between aspheric reference surface 70 and testsurface 38 can be adjusted. Furthermore, this variable spacing can alsobe exploited to perform a fringe scan, which is a conventional method ofanalyzing interference fringes.

The exemplary configuration of holder assembly 72 shown in FIG. 8b issimilar to the exemplary configuration shown in FIG. 8a in thatreference element 76 with aspheric reference surface 70 is held byreference element holder 72H. However, holder assembly 72 of FIG. 8b hasspacers 72S directly vacuum-deposited at three locations on asphericreference surface 70. Spacers 72S are 1 to 3 μm in thickness, thisthickness being identical at all three locations. Furthermore, spacers72S are provided so that they trisect the circumference about an axis Axin the vertical direction of the paper surface in FIG. 8b. Test surface38 is mounted on (three) spacers 72S. The spacing between asphericreference surface 70 and test surface 38 can thereby be kept constantand the strain in test surface 38 due to gravity can also be keptconstant. If the exemplary configuration shown in FIG. 8b is employed,it is possible to perform a fringe scan for analyzing interferencefringes by varying laser wavelength, which has the additional benefit ofeliminating the likelihood that the interferometer will be affected bymechanical vibration or the like.

It is preferable that test object 36 be held in holder assembly 72 inthe same manner as it is held in the optical system of which it is anoptical component. It is also preferable that test object 36 be held inholder assembly 72 in the same orientation with respect to gravity as itis held in the optical system of which it is an optical component. Thiswill make it possible to carry out meaningful measurements despitechanges in surface shape which may occur due to the action of strain ontest surface 38 when test object 36 is actually incorporated into anoptical system.

It is also preferable to make the spacing between aspheric referencesurface 70 and test surface 38 less than 1 mm. If this spacing exceeds 1mm, the impact of air fluctuations, vibration, sound, air pressurefluctuations, temperature fluctuations and optical system aberrationsincreases, leading to a deterioration in measurement accuracy. Tofurther improve measurement accuracy, it is preferable to set thespacing between aspheric reference surface 70 and test surface 38 to beless than 100 μm.

In addition, if the spacing between aspheric reference surface 70 andtest surface 38 is fixed as in FIG. 8b, it is preferable to set thisspacing to be less than 10 μm.

Variation on Fourth Embodiment

In the exemplary configuration shown in FIG. 8a and discussed above, thespacing between test surface 38 and aspheric reference surface 70 may bedetected by the following techniques.

Referring now also to FIG. 9, a ninth surface-shape-measuringinterferometer 22I is a variation on the above-described interferometer22H of the fourth embodiment shown in FIG. 7. In interferometer 22I,elements similar in function to elements as those in interferometer 22Hhave been given the same reference numerals and so a description thereofis omitted.

Interferometer 22I shown in FIG. 9 differs from interferometer 22H shownin FIG. 7 in that a shearing interferometer 80 is provided behind testsurface 38 (i.e., at the side opposite from aspheric reference surface70). Shearing interferometer 80 guides light from a white light source80S to test surface 38 and aspheric reference surface 70 by way of abeam splitter 80BS. Light reflected by test surface 38 and lightreflected by aspheric reference surface 70 passes through beam splitter80BS, and is horizontally displaced by a birefringent member 80BR. Thelatter may be, for example, a Wollaston prism. The light then passesthrough an analyzer 80A and forms an interference pattern on detector60, such as a CCD. The spacing between test surface 38 and asphericreference surface 70 can be detected by monitoring the change in theinterference pattern on detector 60. In addition, in interferometer 22Ishown in FIG. 9, optical element 36 having test surface 38 is preferablymade of an optically transmissive material such as, for example, quartzor Zerodur.

Fifth Embodiment

Referring now to FIGS. 10a-14, we describe a fifth embodiment accordingto a third aspect of the present invention. FIGS. 10a and 11 a showfirst and second wavefront-aberration-measuring interferometers 22J and22K. FIGS. 12-14b show exemplary configurations of a thirdwavefront-aberration-measuring interferometer 22L according to the fifthembodiment.

Interferometers 22J, 22K, and 22L, respectively shown in FIGS. 10a, 11a, and 12, are not “Fizeau” aspheric-surface-shape-measuringinterferometers for measuring the surface shape of a test surface 38 ofa test object 36 previously removed from an optical system of which itis an optical component, as were interferometers 122, 222, and 22A-22Ishown in FIGS. 1-9. Rather, they are wavefront-aberration-measuringinterferometers for measuring the wavefront aberration produced by anoptical system. Note that for the sake of convenience, the term“interferometer” is used to refer to either anaspheric-surface-shape-measuring interferometer, awavefront-aberration-measuring interferometer, or to both, when themeaning is clear from context.

The wavefront-aberration-measuring interferometers 22J-22L according tothe fifth embodiment of the present invention use light corresponding toan exposure wavelength in the soft X-ray region to measure wavefrontaberration of a projection optical system.

Referring to FIGS. 10a-11 c, the principle of thewavefront-aberration-measuring interferometer of the fifth embodimentaccording to a second aspect of the present invention is now described.

With reference to FIG. 10a, light from a synchrotron orbital radiation(hereinafter “SOR”) undulator (not shown) passes through a spectroscope(not shown) to form quasimonochromatic light 84 having a wavelengtharound 13 nm. Light 84 is condensed by a condenser mirror 64 and isincident a first pinhole plate 86. First pinhole plate 86 has anaperture (pinhole) 86 o of a size smaller than the size of the Airy diskas determined from the numerical aperture on the incident side (firstpinhole plate 86 side) of an optical system 37 under test. The size ofthe Airy disk is given by 0.6 λ/NA, where NA is the incident-sidenumerical aperture of optical system 37, and λ is the wavelength ofquasimonochromatic light 84.

Light having a wavefront which can be regarded as that of an idealspherical wavefront will exit first pinhole plate 86. Light from firstpinhole plate 86 is then incident optical system 37, and then arrives ata pinhole plate 88 having an aperture 88 o arranged at an image plane IPof optical system 37. First pinhole plate 86 and second pinhole plate 88are arranged at locations made mutually conjugate by optical system 37,i.e., at locations corresponding to what would be an object point and animage point if optical system 37 were actually used to image an object.

Referring to FIG. 10b, pinhole plate 88 comprises a semitransparent film88F provided on a substrate 88S which is optically transmissive at thewavelength of emitted quasimonochromatic light 84, and aperture 88 owherein semitransparent film 88F is not provided. Accordingly, a portionof the wavefront incident pinhole plate 88 is transmitted withoutalteration of the wavefront, and another portion undergoes diffractionat aperture 88 o. Accordingly, if the size of aperture 88 o issufficiently small, the light diffracted at aperture 88 o can beregarded as an ideal spherical wavefront.

Referring again to FIG. 10a, detector 60 is arranged on the exit side ofpinhole plate 88 (i.e., at the side thereof opposite from optical system37). Interference fringes are formed on detector 60 due to interferencebetween the ideal spherical wavefront from aperture 88 o and thetransmitted wavefront from semitransparent film 88F. The transmittedwavefront from semitransparent film 88F corresponds in shape to thewavefront aberration of optical system 37. The interference fringes ondetector 60 assume a shape corresponding to the deviation of thistransmitted wavefront from an ideal spherical wavefront (ie., thewavefront from aperture 88 o). Accordingly, the wavefront aberration ofoptical system 37 can be determined by analyzing, in a computer CUelectrically connected to detector 60, the interference fringes detectedby detector 60.

FIGS. 11a is a fourth wavefront-aberration-measuring interferometer 22Kemploying an SOR undulator light source and which is a variation ofwavefront-aberration-measuring interferometer 22J of FIG. 10a. Note thatin FIGS. 11a-11 c, elements similar in function to elements appearing inFIGS. 10a and 10 b are given the same reference numerals as in FIGS. 10aand 10 b. Interferometer 22K makes use of a measurement technique ofhigher precision than that of interferometer 22J. Interferometer 22K inFIG. 11a differs from interferometer 22J in FIG. 10a in that a seconddual hole plate 90 is arranged in place of second pinhole plate 88, anda diffraction grating 62 is inserted between first pinhole plate 86 andsecond dual hole plate 90.

FIG. 11b shows the constitution of second dual hole plate 90, and FIG.11c is a diagram for explaining the functions of diffraction grating 62and second dual hole plate 90. Referring to FIG. 11b, second dual holeplate 90 has microscopic aperture 90 o that functions as a pinhole, andan aperture 92 that is larger than pinhole 90 o. Pinhole 90 o andaperture 92 are formed such that if second dual hole plate 90 is at thelocation of image plane IP of optical system 37, pinhole 90 o ispositioned in the optical path of the zeroeth-order peak P0 of thediffraction pattern produced by diffraction grating 62. In addition,aperture 92 is positioned in the optical path of a first-order peak P1of the diffraction pattern produced by diffraction grating 62, as shownin FIG. 11c.

Accordingly, zeroeth-order peak P0 is diffracted by pinhole 90 o,forming an ideal spherical wavefront 45 which then proceeds to detector60. In addition, a wavefront 45′ associated with first-order peak P1,which contains information about the wavefront aberration of opticalsystem 37, passes through aperture 92 without alteration, and proceedsto detector 60. At this time, zeroeth-order peak PO and first-order peakP1 have wavefronts 45 and 45′, respectively, corresponding to thewavefront aberration of optical system 37. Wavefront 45 of the lightthat passes through pinhole 90 o, is converted to an ideal sphericalwavefront. However, wavefront 45′ passing through aperture 92 does notundergo any significant amount of diffraction, and so has a wavefrontshape corresponding to the wavefront aberration of optical system 37.Accordingly, interference fringes due to interference between idealspherical wavefront 45 from pinhole 90 o and measurement wavefront '45from aperture 92 are formed on detector 60. The profile of theinterference fringes formed on detector 60 will correspond to thedeviation of the measurement wave from an ideal spherical wavefront 45,and wavefront 45′ containing aberration information of optical system 37can be determined by analyzing these interference fringes, as in thecase for interferometer 22J of FIG. 10a.

With continuing reference to FIG. 11a, a fringe scan for high-precisionmeasurement can be performed by moving diffraction grating 62 byoperatively connecting the latter to a diffraction grating driving unitDU. In interferometer 22K, diffraction grating 62 is shown arranged inthe optical path between optical system 37 and second dual hole plate90. However, diffraction grating 62 may be arranged in the optical pathanywhere between first pinhole plate 86 and second dual hole plate 90.For example, it is possible to arrange diffraction grating 62 in theoptical path between first pinhole plate 86 and optical system 37. Inaddition, while the above-described embodiment of interferometer 22Kemployed two diffraction orders PO and P1 of the diffraction patternproduced by diffraction grating 62, the present invention is not limitedto two such orders or of combinations of the zeroeth-order andfirst-order.

Referring now to FIG. 12, a fifth wavefront-aberration-measuringinterferometer 22L, which represents a fifth embodiment according to thesecond aspect of the present invention for measuring the wavefrontaberration of an optical system 37 based on the principle explainedabove with reference to FIGS. 10a-11 c, is now described. In FIG. 12,elements similar in function to elements appearing in FIGS. 10a-11 c aregiven the same reference numerals as in FIGS. 10a and 10 b.

In interferometers 22J and 22K shown in FIGS. 10a and 11 a, theaberration of optical system 37 can only be measured at one point inimage plane IP. To accurately ascertain the aberration of an opticalsystem, it is necessary to measure a plurality of image points. Tomeasure a plurality of image points in interferometers 22J and 22K, oneconceivable method of performing measurements would involve moving firstpinhole plate 86 and second pinhole plate 88, or second dual hole plate90, to a number of prescribed positions. In this case, since thepinholes are extremely small, there is a risk that the pinholes will beaffected by the vibration of the movement mechanism that moves thepinholes, and that particularly for pinholes on the image side, it willnot be possible to make light pass through these pinholes stably. Thismakes good measurements extremely problematic. In addition, if pinholesare moved, it becomes difficult to measure the pinhole positions withgood accuracy. Further, there is a risk that the accuracy with whichaberration (particularly distortion), is measured will no longer besufficient, particularly for image points.

In interferometer 22L, a first pinhole array plate 93, wherein pinholesare arrayed in two dimensions, is used in place of first pinhole plate86 of interferometer 22K shown in FIG. 11a.

Referring to FIG. 12, light from an SOR undulator (not shown) passesthrough an analyzer (not shown) to form quasimonochromatic light 84having of wavelength around 13 nm. This light is condensed by condensermirror 64 and is incident first pinhole array plate 93. Unlikewavefront-aberration-measuring interferometers 22J and 22K shown inFIGS. 10a and 11 a, interferometer 22L shown in FIG. 12 is constitutedsuch that light is incident the image plane IP side, not the objectplane OP side, of optical system 37, the reason for which is discussedbelow.

Turning briefly to FIG. 13a, first pinhole array plate 93 comprises anarray or matrix of pinhole apertures (pinholes) 93 o of a size wellsmaller than the size of the Airy disk 0.6 λ/NA, as determined from thenumerical aperture (imagewise numerical aperture) NA at the incidentside of optical system 37. The positions of pinholes 93 o correspond tothe locations of image points of optical system 37 for which measurementof wavefront aberration is desired.

Returning now to FIG. 12, condenser mirror 64 is provided on a condensermirror stage 66, which is capable of movement parallel to image plane IPof optical system 37. By moving condenser mirror stage 66, any desiredpinhole 93 o on first pinhole array plate 93 can be selectivelyilluminated. An illuminated pinhole 93 o corresponds to a measurementpoint. Furthermore, the position at which quasimonochromatic light 84 isincident first pinhole array plate 93 will change with the movement ofcondenser mirror stage 66. In addition, it is also possible tocollectively illuminate a plurality of pinholes 93 o on first pinholearray plate 93 instead of, or in addition to, illuminating just one ofthe pinholes. Nonetheless, in the description below, it is assumed forthe sake of convenience, that only one pinhole 93 o is illuminated.

Referring now also to FIG. 13b, second dual hole array plate 94 islocated in object plane OP, ie., arranged at the position at whichoptical system 37 images first pinhole array plate 93. Second dual holearray plate 94 has a plurality of pinhole apertures (pinholes) 94 oprovided in a matrix at positions at which the plurality of pinholes 93o of first pinhole array plate 93 are imaged, and a plurality ofapertures 95 provided in a matrix such that each is separated by aprescribed distance from each of the plurality of pinholes 94 o.Furthermore, each of the plurality of pinholes 94 o has the samefunction as pinholes 90 o in FIG. 11b, and each of the plurality ofapertures 95 has the same function as aperture 92 in FIG. 11b.

Referring again to FIG. 12, light having a wavefront 45, which can beregarded as that of an ideal spherical wavefront, exits an illuminatedpinhole 93 o, and is incident optical system 37. This light passesthrough optical system 37 and is diffracted by diffraction grating 62arranged between optical system 37 and object plane IP. Zeroeth-orderpeak P0 (not shown in FIG. 12) of the diffraction pattern arrives atpinhole 94 o on second dual hole array plate 94 corresponding to theilluminated pinhole 93 o on first pinhole array plate 93. First-orderpeak P1 (not shown on FIG. 12) of the diffraction pattern arrives ataperture 94 o on second dual hole array plate 94 corresponding to theilluminated pinhole 93 o on first pinhole array plate 93. Light thatpasses through pinhole 94 o and the light that passes through aperture95 mutually interfere.

With continuing reference to FIG. 12, detector 60, is attached to adetector stage 68 which is capable of movement parallel to object planeOP, is arranged at the exit side of second dual hole array plate 94.Detector stage 68 is constituted so that it is linked with and moveswith condenser mirror stage 66, and such that only pinhole 94o andapertures 95, corresponding to illuminated pinhole 93 o, can be seenfrom detector 60. Accordingly, the interference fringes due to the lightonly from pinhole 94 o and aperture 95, corresponding to the illuminatedpinhole 93 o, are formed on detector 60. By analyzing these interferencefringes, the wavefront aberration at image plane IP locationcorresponding to illuminated pinhole 93 o can be determined.

In interferometer 22L of FIG. 12, first pinhole array plate 93 andsecond dual hole array plate 94 are physically grounded (i.e., securedso as to be stationary) with respect to optical system 37. Thus, stablemeasurements can be performed without being affected by vibrationscaused by the movement of stages 66, 68 during actual measurements.

First pinhole array plate 93 is mounted on a vertical stage 67, which iscapable of causing first pinhole array plate 93 to move in jogged (i.e.,incremental) fashion in a direction parallel to the optical axis ofoptical system 37. Vertical stage 67 is preferably secured to the sameframe (not shown) that supports optical system 37. In addition, seconddual hole array plate 94 is mounted on an XY stage 69, which is capableof causing second dual hole array plate 94 to move in jogged fashionwithin object plane OP of optical system 37. XY stage 69 is attached tothe abovementioned frame by way of a piezoelectric element. Adjustmentof focus can be performed by using vertical stage 67 to move firstpinhole array plate 93. If there is distortion in optical system 37, XYstage 69 can be used to align the position of pinhole 94 o. Furthermore,a length measuring interferometer or other such microdisplacement sensor(not shown) may be provided on XY stage 69, permitting distortion inoptical system 37 to be measured based on the output from themicrodisplacement sensor. Furthermore, in the present embodiment, thepositions of the plurality of pinholes 93 o of first pinhole array plate93 and the plurality of pinholes 94 o of second dual hole array plate 94are accurately measured beforehand using a coordinate measuringinterferometer.

Although the position of pinhole 94 o is moved in interferometer 22L,this pinhole can be positioned with good accuracy since the stroke ofthis movement is small. Furthermore, interferometer 22L is constitutedsuch that pinhole 94 o, on the object plane OP side of optical system 37is moved when optical system 37 has a reduction magnification of −1/β.Thus, the positioning accuracy of pinhole 94 o can be relaxed by thefactor |1/β| as compared with the case in which pinhole 93 o, on theimage plane IP side of optical system 37, is moved.

Interferometer 22L is not constituted so that pinhole 93 o is moved andthe amount of movement of pinhole 94 o is in a range wherein positioningaccuracy can be maintained. Thus, stable measurement can be achieved,and the measurement accuracy of aberration, particularly distortion, atthe imaged location can be made sufficient.

In interferometer 22L shown in FIG. 12, the plurality of pinholes 93 ocorresponding to positions for measurement of the wavefront aberrationof optical system 37 are shown arranged in a matrix. However, thearrangement of pinholes 93 o is not limited to a typical square orrectangular matrix. For example, referring to FIG. 14a, if the field(exposure field) EF of optical system 37 is arcuate, as shown in FIGS.14a and 14 b, then a pinhole plate 93′ having pinholes 93 o may bearranged with a prescribed spacing at an object height (image height) ofthe same height as that of optical system 37. Also, as shown in FIG.14b, the arrangement of the pinholes 94 o and apertures 95 in seconddual hole array plate 94′ will have to be prealigned with pinholes 93 oof the first pinhole array plate 93.

While diffraction grating 62 in interferometer 22L of FIG. 12 isarranged in the optical path between optical system 37 and second dualhole array plate 94, diffraction grating 62 may also be arranged in theoptical path between first pinhole array plate 93 and second dual holearray plate 94. For example, it is possible for diffraction grating 62to be arranged between first pinhole array plate 93 and optical system37. In addition, while interferometer 22L shown in FIG. 12 employs twopeaks of the diffraction pattern produced by diffraction grating 62,i.e., zeroeth-order peak P0 of the diffraction pattern and first-orderpeak P1 of the diffraction pattern, the present invention is not limitedto employment of two such peaks, or of employment of combinations ofzeroeth-order and first-order peaks.

Sixth Embodiment

Referring now to FIGS. 15a and 15 b, a fourthwavefront-aberration-measuring apparatus 22M of a sixth embodimentaccording to the present invention is now described. Apparatus 22M usesa soft X-ray exposure wavelength to measure the wavefront aberration ofan optical system 37. Note that in FIGS. 15a and 15 b, elements similarin function to elements appearing in FIGS. 10a-14 b are given the samereference numerals as in FIGS. 10a-14 b.

Referring to FIG. 15a, light from an SOR undulator (not shown) passesthrough an analyzer (not shown) to form quasimonochromatic light 84having a wavelength around 13 nm, which is condensed by a condensermirror 64 and is incident first pinhole plate 86. First pinhole plate 86has an aperture of a size well smaller than the size of the Airy disk,0.6 λ/NA, where λ is the wavelength of quasimonochromatic light 84 andNA is the numerical aperture on the incident side (first pinhole plate86 side) of optical system 37. Accordingly, the light that exits firstpinhole plate 86 can be regarded as having the wavefront of an idealspherical wavefront.

In apparatus 22M, a second Hartmann plate 96 having a plurality ofapertures 96 o, as shown in FIG. 15b, is arranged between the locationof image plane IP of optical system 37 (a location made conjugate tofirst pinhole plate 86 by optical system 37) and optical system 37.

Returning to FIG. 15a, the light beam from first pinhole plate 86, uponexiting optical system 37, forms, due to the action of the plurality ofapertures 96 o of second Hartmann plate 96, a plurality of ray groups RGthat are the same in number as the number of apertures 96 o. Ray groupsRG then proceed to image plane IP of optical system 37. Ray groups RGconverge at image plane IP of optical system under test 37 and arrive atdetector 60 in a divergent state. If the plane of the pupil (not shown)of optical system under test 37 is subdivided into a plurality ofsections, ray groups RG that pass through the plurality of apertures 96o on second Hartmann plate 96 respectively correspond to rays passingthrough each such pupil section. As a result, the lateral aberration ofoptical system 37 can be determined if the position at which each of raygroups RG arrives at detector 60 is detected. The wavefront aberrationof optical system 37 can be determined from this lateral aberration.

In apparatus 22M, the plurality of apertures 96 o provided on secondHartmann plate 96 are arranged in a matrix as shown in FIG. 15b.However, the present invention is not limited to this arrangement. Inaddition, while in apparatus 22M second Hartmann plate 96 is arrangedbetween optical system 37 and image plane IP second Hartmann plate 96may also be located between first pinhole plate 86 and image plane IP,it being possible, for example, for second Hartmann plate 96 to bearranged in the optical path between first pinhole plate 86 and opticalsystem 37.

Seventh Embodiment

Referring now to FIGS. 16a-16 c, a fifth wavefront-aberration-measuringinterferometer 22N of a seventh embodiment according to a third aspectof the present invention is described. Interferometers 22J, 22K, 22L,and 22M of the fifth and sixth embodiments discussed above arewavefront-aberration-measuring interferometers which employ an SORundulator (not shown) as a light source. Although accuracy can be madeextremely high if an SOR undulator is used as a light source, theapparatus itself becomes excessively large, and it is generallyextremely difficult to use in a factory. Thus, referring to FIG. 16a, ininterferometer 22N discussed in further detail below, a laser plasmaX-ray (hereinafter “LPX”) source 21 is used in place of an SOR undulatoras light source. LPX source 21 generates high-temperature plasma from atarget 25 when high-intensity pulsed laser light is focused on target25. X-rays present within this plasma are then used. In interferometer22N, light emitted from LPX source 21 is divided into spectralcomponents by a spectroscope (not shown), and light 27 of only aprescribed wavelength (e.g., 13 nm) is extracted. Light 27 is used asthe light for wavefront-aberration-measuring interferometer 22N.

The intensity of LPX source 21 is smaller than that of the SOR undulatorby an order of magnitude. Consequently, in interferometer 22N, firstpinhole plate 86, which had only a single aperture in interferometers22J, 22K, 22L, and 22M of the fifth and sixth embodiments shown in FIGS.10a-15 b and discussed above, is replaced with a first pinhole clusterplate 87. The latter includes a plurality of pinhole clusters 87 c, eachof which contains a plurality of pinholes 87 o clustered together in amicrolocation, as shown in FIG. 16b.

Referring again to FIG. 16a, in LPX source 21, a laser light source 23supplies high-intensity pulsed laser light of a wavelength in the rangefrom the infrared region to the visible region. Laser light source 23may be, for example, a YAG laser excited by a semiconductor laser, anexcimer laser, or the like. This laser light is condensed by a condenseroptical system 29 onto target 25. Target 25 receives the high-intensitylaser light, rises in temperature and is excited to the plasma state,and emits X-rays 27 during transitions to a lower potential state. Bypassing X-rays 27 through a spectroscope (not shown), quasimonochromaticlight 27 only of wavelength 13 nm is extracted, which is then acted onby condenser mirror 64 and irradiates a pinhole cluster 87 c on firstpinhole cluster plate 87.

Referring again to FIG. 16b, first pinhole cluster plate 87 has pinholeclusters 87 c, each of which comprises a plurality of pinholes 87 oclustered in a microlocation at a position for which the wavefrontaberration of optical system 37 is to be measured. Note that in FIG.16b, pinhole cluster 87 c is shown as having only four pinholes 87 o.However, pinhole cluster 87 c preferably actually comprises one hundredor more pinholes 87 o. Pinholes 87 o are of a size much smaller than thesize of the Airy disk 0.6 λ/NA, where λ is the wavelength ofquasimonochromatic light 27 and NA is the numerical aperture on theincident side (first pinhole cluster plate 87 side) of optical system37. In addition, FIG. 16b shows an exemplary schematic arrangementwherein a plurality of pinhole clusters 87 c are formed on first pinholecluster plate 87. In practice the positions at which pinhole clusters 87c are formed to correspond to the positions of object points of opticalsystem 37 for which measurement is desired.

Returning to FIG. 16a, the entire region of one pinhole cluster 87 c onfirst pinhole cluster plate 87 is illuminated by quasimonochromaticlight 27. A plurality of ideal spherical wavefronts are generated fromthe numerous pinholes 87 o of the illuminated pinhole cluster 87 c. Theplurality of ideal spherical wavefronts passes through optical system37, and then proceeds to and converges at image plane IP of opticalsystem 37, which position is made conjugate to first pinhole clusterplate 87 by optical system 37.

Although not shown in FIGS. 16a-16 c, in interferometer 22N one ofpinhole clusters 87 c on first pinhole cluster plate 87 is selectivelyilluminated, just as in the case of interferometers 22J, 22K, and 22L ofthe fifth embodiment, discussed above.

In interferometer 22N diffraction grating 62 is arranged between opticalsystem 37 and the location of the image plane IP of optical system 37.The light that exits optical system 37 and passes through diffractiongrating 62 is diffracted by diffraction grating 62 and proceeds to asecond dual hole cluster plate 89.

FIG. 16c shows a preferred constitution of second dual hole clusterplate 89. Second dual hole cluster plate 89 has pinhole cluster 89 ccomprising a plurality of pinholes 89 o provided in one-to-onecorrespondence with the pinholes 87 o of which plurality of pinholeclusters 87 c on first pinhole cluster plate 87 are each comprised, anda plurality of apertures 89 a provided in one-to-one correspondence withthe plurality of pinhole clusters 87 c. In other words, one aperture 89a corresponds to one pinhole cluster 87 c comprising a plurality ofpinholes 87 o.

At this time, if second dual hole cluster plate 89 is arranged at imageplane IP, then plurality of pinhole clusters 89 c and plurality ofapertures 89 a will be positionally related so that pinhole cluster 89 cis positioned in the optical path of the zeroeth-order peak P0 of thediffraction pattern produced by diffraction grating 62, and so thataperture 89 a is positioned in the optical path of first-order peak P1of the diffraction pattern produced by diffraction grating 62.

Accordingly, the ideal spherical wavefronts from pinhole cluster 87 c onfirst pinhole cluster plate 87 pass through optical system 37 and arethen diffracted by diffraction grating 62. Of the light produced by thisdiffraction, zeroeth-order peak P0 of the diffraction pattern arrives atthe pinhole cluster 89 c on second dual hole cluster plate 89, whichcorresponds to illuminated pinhole cluster 87 c. In addition,first-order peak P1 of the diffraction pattern arrives at the aperture89 a on second dual hole cluster plate 89, which corresponds toilluminated pinhole cluster 87 c. Zeroeth-order peak P0 of thediffraction pattern and first-order peak P1 of the diffraction patternhave wavefronts corresponding in shape to the wavefront aberration ofoptical system 37. Zeroeth-order peak P0 of the diffraction pattern isdiffracted by pinhole cluster 89 c as it passes therethrough and isconverted to a second group of ideal spherical wavefronts. First-orderpeak P1 of the diffraction pattern passes through aperture 89 a andexits therefrom without being diffracted. The light from the secondideal spherical wavefront group and the light from aperture 89 amutually interfere.

Accordingly, interference fringes due to interference between the idealspherical wavefront group from pinhole cluster 89 c and the wavefrontfrom aperture 89 a are formed on detector 60 arranged on the exit sideof second dual hole cluster plate 89 (i.e., on the side of second dualhole cluster plate 89 opposite from optical system 37). Furthermore, theinterference fringes on detector 60 form a shape corresponding to thedeviation from an ideal spherical wavefront of the wavefront that passesthrough optical system 37. The wavefront aberration of optical system 37can be determined by analyzing these interference fringes via computerCU electrically connected to detector 60, just as in the previouslymentioned embodiments.

Furthermore, although not shown in FIG. 16a, detector 60 is constitutedso as to be capable of movement parallel to image plane IP of opticalsystem 37 so that it can be made to selectively receive the light frompinhole cluster 89 c and aperture 89 a corresponding to illuminatedpinhole cluster 87 c, just as in interferometers 22J, 22K, and 22L ofthe fifth embodiment, discussed above. As a result, wavefront aberrationcan be measured at a plurality of measurement points within object planeOP of optical system 37.

The seventh embodiment of the present invention as described above canprovide a wavefront-aberration-measuring interferometer 22N that can beused even in an ordinary factory.

Furthermore, while diffraction grating 62 in interferometer 22N of theseventh embodiment shown in FIG. 16a is arranged in the optical pathbetween optical system 37 and second dual hole cluster plate 89,diffraction grating 62 may also be arranged in the optical path betweenfirst pinhole cluster plate 87 and second dual hole cluster plate 89. Itbeing possible, for example, to arrange diffraction grating 62 in theoptical path between first pinhole cluster plate 87 and optical system37. Also, while interferometer 22N employs two peaks of the diffractionpattern produced by diffraction grating 62 (zeroeth-order peak P0 andfirst-order peak P1) the present invention is not limited to employmentof two such peaks or of employment of combinations of the zeroeth-orderand first-order peaks.

Eighth Embodiment

Referring now to FIGS. 17a and 17 b, an eighth embodiment according to athird aspect of the present invention is described. Interferometer 22Nof the seventh embodiment shown in FIG. 16a and described above employedpinhole clusters 87 c, 89 c provided with a plurality of pinholes 87 o,89 o in prescribed microlocations. However, a pinhole row plate 97 maybe used, wherein plate 97 includes a plurality of a pinhole rows 97Rwherein a plurality of pinholes 97 o are arranged unidimensionally in aprescribed direction, as shown in FIG. 17a. In this case, first pinholerow plate 97 is provided with a plurality of rows 97R of pinholes 97 oarrayed in matrix-like fashion so as to correspond to a plurality ofmeasurement points in object plane OP or image plane IP of opticalsystem 37. Although FIG. 17a shows a pinhole row 97R having only fourpinholes 97 o, an actual pinhole row 97R comprises 100 or more pinholes97 o. Pinholes 97 o are of a size smaller than the Airy disk 0.6 λ/NA,where λ is the wavelength of quasimonochromatic light 84 and NA is thenumerical aperture on the incident side of optical system 37 (i.e., onthe side thereof at which first pinhole row plate 97, which here takesthe place of first pinhole cluster plate 87 shown in FIG. 16a, ispresent).

Referring back and forth between FIGS. 16a-16 c and FIGS. 17a-17 b, iffirst pinhole row plate 97 is used in place of first pinhole clusterplate 87, then a second dual hole row plate 99 should be used in placeof second dual hole cluster plate 89. Second dual hole row plate 99 hasa plurality of pinhole rows 99R, each of which comprises a plurality ofpinholes 99 o provided in one-to-one correspondence with pinholes 97 oof which pinhole rows 97R on first pinhole row plate 97 are eachcomprised. In addition, plate 99 has a plurality of apertures 99 aprovided in one-to-one correspondence with plurality of pinhole rows 97o. Furthermore, each of the plurality of pinhole rows 99R comprisesnumerous pinholes 99 o arrayed unidimensionally in a prescribeddirection. In addition, one aperture 99 a corresponds to one pinhole row97R comprising plurality of pinholes 97 o.

Employment of a pinhole row 97R, 99R thus comprising a plurality ofpinholes 97 o, 99 o arrayed unidimensionally in a prescribed directionmakes it possible to reduce noise caused by the intermixing of lightamong the plurality of pinholes 92 o, 94 o, 93 o, 95 o, 96 o, 87 o, 89o, and measurement accuracy can thereby be further improved.

It is also preferable to make the pitch of the plurality of pinholesarrayed unidimensionally in a prescribed direction be 10 to 25 times theradius of the Airy disk 0.6 λ/NA as determined by the numerical apertureon the first pinhole row plate 97 side of optical system 37. It isfurther preferable to make it approximately 16 to 20 times this Airydisk radius.

Ninth Embodiment

Referring now to FIGS. 18a and 18 b, we describe a ninth embodimentaccording to a third aspect of the present invention. It is possible touse slit-shaped apertures 57 s, 59 s in place of pinhole clusters 87 c,89 c in interferometer 22N shown in FIG. 16a and described above. FIGS.18a and 18 b show slit plates 57, 59 provided with pluralities ofslit-shaped apertures 57 s, 59 s.

In describing the use of first slit plate 57 and second dual slit plate59 in place of first pinhole cluster plate 87 and second dual holecluster plate 89, to reference is made back and forth between FIGS.16a-16 c and FIGS. 18a-18 b.

In FIG. 18a, first slit plate 57 is provided with a plurality ofslit-shaped apertures 57 s arrayed in matrix-like fashion so as tocorrespond to a plurality of measurement points in object plane OP imageplane IP of optical system 37. Furthermore, the slit shape mentioned inthe present embodiment refers to a shape extending unidimensionally in aprescribed direction, the overall shape hereof not being limited torectangular. In addition, the width in the latitudinal direction ofslit-shaped aperture 57 s is of a size well smaller than the size of theAiry disk 0.6 λ/NA, where λ is the wavelength of quasimonochromaticlight 27 and NA is the by numerical aperture on the incident side (onthe side of first slit plate 57, which here corresponds to first pinholecluster plate 87 in FIG. 16a) of optical system 37. Upon illumination ofa slit-shaped aperture 57 s, the wavefront emitted therefrom will besuch that its cross section in the short direction of the slit-shapedaperture 57 s is the same as that of an ideal spherical wavefront (i.e.,this then can be said to represent a one-dimensional ideal sphericalwavefront).

If first slit plate 57 shown in FIG. 18a is used in place of firstpinhole cluster plate 87 shown in FIG. 16b, then second dual slit plate59 shown in FIG. 18b should be used in place of second dual hole clusterplate 89. Second dual slit plate 59 comprises a plurality of slit-shapedapertures 59 s provided in one-to-one correspondence with the pluralityof slit-shaped apertures 57 s on first slit plate 57, and a plurality ofapertures 59 a provided in one-to-one correspondence with the pluralityof slit-shaped apertures 57 s on first slit plate 57.

In the ninth embodiment of the invention, slit plates 57, 59 shown inFIGS. 18a and 18 b are incorporated in wavefront-aberration-measuringinterferometer 22N of the seventh embodiment shown in FIG. 16a.Operation in this case is as follows.

First, one of the plurality of slit-shaped apertures 57 s first slitplate 57 corresponding to a desired measurement point is illuminatedwith light 27 from LPX source 21. The wave emitted from the illuminatedslit-shaped aperture 57 s is such that a one-dimensional ideal sphericalwavefront is generated in the short direction of slit-shaped aperture 57s. This one-dimensional ideal spherical wavefront passes through opticalsystem 37 and is diffracted by diffraction grating 62. Zeroeth-orderpeak P0 of the diffraction pattern arrives at the correspondingslit-shaped aperture 59 s on second dual slit plate 59, and first-orderpeak P1 of the diffraction pattern arrives at aperture 59 a on seconddual slit plate 59.

Furthermore, a one-dimensional ideal spherical wavefront is generated inthe short direction of the corresponding slit-shaped aperture 59 s onsecond dual slit plate 59, and a wavefront corresponding in shape to thewavefront aberration of optical system 37 passes through aperture 59 a.The wavefront of the one-dimensional ideal spherical wavefront and thewavefront from the aperture 59 a mutually interfere and forminterference fringes on detector 60. The wavefront aberration of opticalsystem 37 can be measured by analyzing these interference fringes incomputer CU. Furthermore, it is possible in this ninth embodiment thatmeasurement accuracy will lower in a direction parallel to the longdirection of slits 57 s, 59 s. If this should be the case, all that needbe done to rectify this is to arrange slit plates 57, 59 and opticalsystem 37 such that they are rotatable relative to one another, or toprovide a plurality of slit-shaped apertures 57 s, 59 s having longdirections in mutually different orientations in place of theslit-shaped apertures 57 s, 59 s shown in FIGS. 18a and 18 b.

Thus, by using slit-shaped apertures 57 s, 59 s, it is possible tofurther increase light flux as compared with cases wherein pinholeplates having a single pinhole, or a pinhole cluster or a pinhole rowcomprising a plurality of pinholes, are used. This constitutioncorresponds to a shearing interferometer.

Also, while second dual slit plate 59 makes use of two peaks of thediffraction pattern produced by diffraction grating 62 (zeroeth-orderpeak P0 and first-order peak P1), the present invention is not limitedto employment of two such peaks or of employment of combinations of thezeroeth-order and first-order peaks thereof.

Tenth Embodiment

Referring to FIG. 19, we describe a sixth wavefront-aberration-measuringinterferometer 22P of a tenth embodiment according to a third aspect ofthe present invention.

Interferometer 22P is a variation on the above-discussed interferometers22M, 22N in the sixth embodiment shown in FIGS. 15a-16 c. An LPX source21 is used in interferometer 22P of the tenth embodiment in place of theSOR undulator light source (not shown) that was used in interferometers22M, 22N of the sixth embodiment.

Referring to FIG. 19, in LPX source 21, laser light source 23 suppliespulsed laser light of a wavelength in the range from the infrared regionto the visible light region. Laser light source 23 may be, for example,a YAG laser excited by a semiconductor laser, an excimer laser, or thelike. This laser light is condensed by condenser optical system 29 ontotarget 25. Target 25 receives the high-intensity laser light, rises intemperature and is excited to the plasma state, and emits X-rays 27during transitions to a lower potential state. By passing X-rays 27through a spectroscope (not shown), quasimonochromatic light 27 only ofwavelength 13 nm is extracted, which is then acted on by condensermirror 64 and irradiates a pinhole plate 31.

Pinhole plate 31 has a single aperture much larger (i.e., ten or moretimes) than the diameter of the Airy disk 0.6 λ/NA, where λ is thewavelength of quasimonochromatic light 27 and NA is the numericalaperture on the incident side (pinhole plate 31 side) of optical system37. Here, so long as aperture 31 o of pinhole plate 31 can beilluminated such that there is uniform illuminance within object planeOP of optical system 37 and such that there is uniform illuminancewithin the cross section of the light beam incident pinhole plate 31,there is no need to make the size of the aperture of pinhole plate 31smaller than the Airy disk, as is the case for the above-describedembodiments.

In interferometer 22P, illumination is such that there is uniformilluminance within object plane OP and within the cross section of thelight beam incident pinhole plate 31. Accordingly, the pinhole plate 31which is used can have a large aperture 31 o such as has been described.

As in the case in the above-described embodiments, in interferometer22P, light exiting from aperture 31 o of pinhole plate 31 can beregarded as having an ideal spherical wavefront.

As in the case in interferometer 22M, in interferometer 22P, secondHartmann plate 96 (see FIG. 15b) having a plurality of apertures 96 o isarranged between image plane IP of optical system 37 (i.e., a locationmade conjugate to pinhole plate 31 by optical system 37) and opticalsystem 37.

With continuing reference to FIG. 19, the light beam from aperture 31 oof pinhole plate 31, upon exiting from optical system 37, forms, due tothe action of the plurality of apertures 96 o of second Hartmann plate96, a plurality of ray groups RG that are the same in number as thenumber of apertures 96 o. Ray groups RG then proceed to image plane IPof optical system 37. Ray groups RG converge at image plane IP andarrive at detector 60 in a divergent state. If the plane of the pupil(not shown) of optical system 37 is subdivided into a plurality ofsections, ray groups RG that pass through the plurality of apertures 96o on second Hartmann plate 96 respectively correspond to rays passingthrough each such section. As a result, the lateral aberration ofoptical system 37 can be determined if the position at which each of theray groups RG arrives at detector 60 is detected. The wavefrontaberration of optical system 37 can then be determined from this lateralaberration using computer CU, as describe above.

Eleventh Embodiment

Referring now to FIGS. 20a and 20 b, a seventhwavefront-aberration-measuring interferometer 22Q in an eleventhembodiment according to a third aspect of the present invention isdescribed.

Although a light source 21 supplying light in the soft X-ray wavelengthregion was used as light source in the above-described interferometers22N-22P in the seventh through tenth embodiments, it may be convenientto use an ordinary laser light source 41 (see FIG. 20a), not an X-raysource 21 (see FIGS. 16a and 19), when assembling and adjusting opticalsystem 37 at an ordinary factory.

FIG. 20a shows wavefront-aberration-measuring interferometer 22Q of thetenth embodiment which uses a non-X-ray laser light source 41. FIGS.20a-23 are intended to assist in explaining the principle of theeleventh embodiment.

Referring to FIG. 20a, in interferometer 22Q, laser light source 41supplies laser light of a prescribed wavelength. This laser light issplit by a beam splitter 74 adjacent light source 41. One of the beamsb1 so split travels by way of two folding mirrors 35 a and 35 b to acondenser lens 39, and is guided to first pinhole plate 86 having asingle pinhole 86 o. First pinhole plate 86 is arranged at the locationof image plane IP of optical system 37. Pinhole 86 o is of a sizesmaller than the diameter of the Airy disk 0.6 λ/NA, where λ is thewavelength of the laser light and NA is the numerical aperture NA on theincident side (first pinhole plate 86 side) of optical system 37.Accordingly, a first ideal spherical wavefront is generated from pinhole86 o of first pinhole plate 86.

The first ideal spherical wavefront from first pinhole plate 86 passesthrough optical system 37 and is guided to second pinhole mirror plate33 arranged at a position conjugate to first pinhole plate 86 by opticalsystem 37.

Referring to FIG. 20b, second pinhole mirror plate 33 comprises anoptically transparent substrate 33S, reflective surface 33R provided onsubstrate 33S, and aperture 33 o, which is a region wherein reflectivesurface 33R is not provided. Furthermore, aperture 33 o of secondpinhole mirror plate 33 is of a size smaller than the diameter of theAiry disk 0.6 λ/NA, where λ is the wavelength of the laser light and NAis the numerical aperture on the exit side (second pinhole mirror plate33 side) of optical system 37.

Returning again to FIG. 20a, light beam b2 produced by splitting at beamsplitter 74 travels by way of a folding mirror 35 c to pass through acondenser lens 49, and is then guided in a condensed state to the rearside of second pinhole mirror plate 33R (i.e., the back thereof, if theside on which reflective surface 33R is applied is taken as the frontthereof), which is arranged in object plane OP of optical system 37.

Accordingly, a second ideal spherical wavefront will be generated atsecond pinhole mirror plate 33 when light beam b2 from the rear side ofsecond pinhole mirror plate 33 passes through aperture 33 o. Inaddition, the light beam that passes through optical system 37 isreflected by reflective surface 33R of second pinhole mirror plate 33.This reflected light has a wavefront corresponding in shape to thewavefront aberration of optical system 37.

The second ideal spherical wavefront from aperture 33 o of secondpinhole mirror plate 33 and the reflected light from reflective surface33R of second pinhole mirror plate 33 arrive at detector 60 by way oflens 47, and form interference fringes on detector 60.

The interference fringes on detector 60 form a shape corresponding tothe deviation from an ideal spherical wavefront of the wavefront thatpasses through optical system 37. The wavefront aberration of opticalsystem 37 can be determined by analyzing these interference fringesusing computer CU, as described above.

In FIGS. 20a and 20 b, which illustrate the principle of thewavefront-aberration-measuring interferometer 22Q of the eleventhembodiment, one prescribed point in object plane OP (or image plane IP)of optical system 37 is used as the measurement point. If a plurality ofmeasurement points are to be measured, then, referring briefly to FIG.21a, first pinhole array plate 61 wherein a plurality of pinholes 61 oare arranged in a prescribed array may be used in place of first pinholeplate 86 of FIG. 20a. In addition, a second pinhole mirror array plate63 having a plurality of pinholes 63 o and a reflective interstitialsurface 63R may be used in place of second pinhole mirror plate 33 shownin FIGS. 16a and 16 b.

Referring now to FIG. 22, an eighth wavefront-aberration-measuringinterferometer 22R, which is a variation onwavefront-aberration-measuring interferometer 22Q of the eleventhembodiment wherein the wavefront aberration of optical system 37 can bemeasured at a plurality of measurement points, is described. In FIG. 22,elements similar in function to elements appearing in FIG. 20a have beengiven the same reference numerals as in FIG. 20a and description thereofwill be omitted here for the sake of convenience.

Referring to FIG. 22 and interferometer 22R, laser light of a prescribedwavelength from laser light source 41 is split by beam splitter 74. Oneof the light beams b1 so split sequentially travels by way of foldingmirror 35 a to condenser lens 39 provided on condenser lens stage 66capable of movement parallel to the image plane of optical system 37,thereafter arriving at first pinhole array plate 61.

Referring back to FIG. 21a, first pinhole array plate 61 has a pluralityof pinholes 61 o arrayed in a matrix. The positions of the plurality ofpinholes 61 o correspond to the positions of measurement points foroptical system 37. Furthermore, each of the plurality of pinholes 61 ois of a size smaller than the diameter of the Airy disk 0.6 λ/NA, whereλ is the wavelength of the laser light and the NA is the numericalaperture on the incident side (first pinhole array plate 61 side) ofoptical system 37. Accordingly, upon being illuminated, pinhole 61 o onfirst pinhole array plate 61 will generate an ideal spherical wavefront.

Returning again to FIG. 22, as a result of moving condenser lens stage66, a desired pinhole 61 o on first pinhole array plate 61 isselectively illuminated. Furthermore, the position at which the laserlight is incident folding mirror 35 a mounted on condenser lens stage 66changes as condenser lens stage 66 is moved. In addition, instead of oneof pinholes 61 o, a plurality of pinholes 61 o may also be collectivelyilluminated.

With continuing reference to FIG. 22, the ideal spherical wavefront fromfirst pinhole array plate 61 passes through optical system 37, and isthen guided to second pinhole mirror array plate 63, located at aposition conjugate to first pinhole array plate 61 by optical system 37.

Referring briefly again to FIG. 21b, second pinhole mirror array plate63 is provided with reflective interstitial surface 63R arranged suchthat plurality of pinholes 63 o form a matrix, no such reflectiveinterstitial surface 63R being provided at the locations of pinholes 63o. Furthermore, each of the plurality of pinholes 63 o of second pinholemirror array plate 63 is of a size smaller than the diameter of the Airydisk 0.6 λ/NA, where λ is the wavelength of the laser light and NA isthe numerical aperture on the exit side (second pinhole mirror arrayplate 63 side) of optical system 37.

Returning now to FIG. 22, light beam b2 produced by splitting at beamsplitter 74 sequentially travels by way of oscillatory folding mirror 45electrically connected to mirror oscillating unit MU, and then by way offolding mirror 35 to a condenser lens 49, and is then guided in acondensed state to the rear side of second pinhole mirror array plate 63(i.e., the side opposite from the side at which reflective interstitialsurface 63R is present), which is arranged in object plane OP of opticalsystem 37.

Accordingly, an ideal spherical wavefront is generated at second pinholemirror array plate 63 when light beam b2 from the rear side of secondpinhole mirror array plate 63 passes through pinhole 63 o. In addition,when the light beam that passes through optical system 37 is reflectedby reflective interstitial surface 63R of second pinhole mirror arrayplate 63, the reflected light will have a wavefront corresponding inshape to the wavefront aberration of optical system 37.

The ideal spherical wavefront from pinhole 63 o of second pinhole mirrorarray plate 63 and the light reflected by reflective interstitialsurface 63R of second pinhole mirror array plate 63 arrive at detector60 by way of another folding mirror 35 d and lens 47, and forminterference fringes on detector 60.

The interference fringes on detector 60 form a shape corresponding tothe deviation from an ideal spherical wavefront of the wavefront thatpasses through optical system 37. The wavefront aberration of opticalsystem 37 can be determined by analyzing these interference fringesusing computer CU, as discussed above.

In interferometer 22R as a variation on the eleventh embodiment shown inFIG. 22, detector 60, along with the optical system which guides thelight from second pinhole mirror array plate 63 to detector 60, andcondenser lens 49 are mounted on Detector stage 68, which is capable ofmovement parallel to object plane OP of optical system 37. Detectorstage 68 is constituted so that it is linked and moves with condenserlens stage 66 discussed above, and only pinhole 63 o, corresponding tothe illuminated pinhole 61 o, can be seen from detector 60. Accordingly,interference fringes are formed on detector 60 due to interferencebetween the light that passes through optical system 37 from illuminatedpinhole 61 o and the diffracted light from pinhole 63 o on secondpinhole mirror array plate 63 corresponding to the illuminated pinhole61 o. Accordingly, the wavefront aberration at the measurement pointwhere the illuminated pinhole 61 o is positioned can be determined byanalyzing these interference fringes.

Stable measurement can also be performed with interferometer 22R in thisvariation on the eleventh embodiment shown in FIG. 22, without beingaffected by vibrations caused by the movement of stages 66, 68 duringmeasurement.

With continuing reference to FIG. 22, first pinhole array plate 61 ismounted on a vertical stage 67, which is capable of causing firstpinhole array plate 61 to move in jogged (i.e., incremental) fashion ina direction parallel to the optical axis of optical system 37. Verticalstage 67 is secured to the same frame that supports optical system 37.In addition, second pinhole mirror array plate 63 is mounted on an XYstage 69, which is capable of causing second pinhole mirror array plate63 to move in jogged fashion within object plane OP of optical system37. XY stage 69 is attached to the abovementioned frame by way of apiezoelectric element. Furthermore, adjustment of focus can be performedby using vertical stage 67 to move first pinhole array plate 61. Ifthere is distortion in optical system 37, XY stage 69 can be used toalign the position of pinhole 63 o.

Furthermore, a length measuring interferometer or other suchmicrodisplacement sensor is preferably provided on XY stage 69,permitting distortion in optical system 37 to be measured based on theoutput from the microdisplacement sensor. Furthermore, in the presentembodiment, the positions of the plurality of pinholes 61 o of firstpinhole array plate 61 and the plurality of pinholes 63 o of secondpinhole mirror array plate 63 are accurately measured beforehand using acoordinate measuring interferometer.

In addition, oscillatory folding mirror 45 in interferometer 22R in thisvariation on the eleventh embodiment shown in FIG. 22 is constituted soas to permit oscillation via mirror oscillation unit MU, the differencein lengths of the optical paths of the two beams produced by beamsplitter 74 changing in accordance with this oscillation. As a result, afringe scan can be executed for high-precision measurement.

Comparative Example

Referring to FIG. 23, wavefront-aberration-measuring interferometer 22Sis a comparative example for illustrating the advantage ofinterferometers 22Q and 22R of the eleventh embodiment. Interferometer22S of the comparative example shown in FIG. 23 employs an ultravioletlaser 41 instead of the SOR undulator light source employed ininterferometer 22J shown in FIG. 10a. As previously mentioned,measurement accuracy increases as the wavelength of the light source ofthe wavefront-aberration-measuring interferometer is shortened. Sincethe wavelength of an ultraviolet laser 41 is approximately 20 timeslonger than the working wavelength of optical system 37, the accuracy ofinterferometer 22S of the comparative example can be expected to be 20times worse than that of interferometer 22J shown in FIG. 10a.

However, in interferometers 22Q and 22R of FIGS. 20a and A22, thereference wavefront is made to travel along an optical path separatefrom the measurement wavefront. Thus, measurement can be performed witha precision higher than is possible with interferometer 22S of thecomparative example shown in FIG. 23. Thus, in interferometers 22Q and22R of the eleventh embodiment, wavefront aberration can be measuredwith high precision without the need to use an X-ray source.

Method of Calibrating Aspheric-Shape-Measuring Interferometer

FIG. 24 is a flowchart for assisting in describing a method forcalibrating an aspheric-surface-shape measuring interferometer of thetype shown in FIGS. 1-7. In the course of this calibration, awavefront-aberration-measuring interferometer of the type shown in FIGS.10a-22 is used to verify the aspheric shape obtained using theaspheric-surface-shape measuring interferometer. This method orvariations thereof can be applied to any of these interferometers forthe sake of convenience, however, we take the example of calibration ofaspheric-surface-shape measuring interferometer 22H of the fourthembodiment shown in FIG. 7 using wavefront-aberration-measuringinterferometer 22J of the fifth embodiment shown in FIG. 10a.

Before executing step S1 in FIG. 24, the aspheric surface under test 38is first machined to a surface accuracy of approximately 10 nm RMS usingwell-known technology.

At step S1 in FIG. 24, the surface shape of the abovementioned aspherictest surface 38 is measured using interferometer 22H of the fourthembodiment of the present invention shown in FIG. 7. Furthermore,interferometer 22H of the fourth embodiment may also be used startingfrom the time when the aspheric surface is first machined. Whenperforming measurements using interferometer 22H, it is preferable tominimize asymmetric systematic errors (errors in reference surface 70)by collecting data at stepped angular rotations obtained by eitherrotating test surface 38 about the optical axis with respect toreference surface 70 in stepwise fashion or rotating reference surface70 about the optical axis with respect to test surface 38 in stepwisefashion, and averaging the data obtained.

At step S2, using the measurement data from step S1, corrective grindingis performed on the aspheric surface 38 so as to make the shape ofaspheric test surface 38 conform to the design data. FIG. 25 shows asmall tool grinding apparatus 400 for performing this correctivegrinding. Referring to FIG. 25, small tool grinding apparatus 400 hasgrinding head 406 provided with a polisher 410 that rotates, and coilspring 414 that applies a prescribed pressure to polisher 414. Aspherictest surface 38 is ground as a result of application of a constant loadin a direction normal to aspheric test surface 38 as optical testelement 36 is rotated. The amount of grinding is proportional to thedwell time of polisher 410 (i.e., the time that polisher 410 remains ata given position and grinds). Furthermore, the shape of test surface 38is measured using interferometer 22H shown in FIG. 7, just as wasperformed at step S1. If the result of measurement is that the measuredaspheric shape differs from the design shape, the shape of test surface38 is again corrected using small tool grinding apparatus 82. Byrepeating this measurement and correction process, the measured asphericshape and the design aspheric shape can be made to coincide.

At step S3, optical element 36 having test surface 38 shaped as a resultof the operations at step S2 is assembled in the optical system 37 ofwhich it is an optical component.

At step S4, the wavefront aberration of the optical system 37 assembledin step S3 is measured. In connection with the measurement of thiswavefront aberration, a PDI (point diffraction interferometer) employingan SOR (synchrotron orbital radiation) undulator light source, such asin interferometer 22J shown in FIG. 10a, is used. Since the measurementwavelength of interferometer 22J is short, at about 13 nm, the wavefrontaberration of the optical system can be measured with high precision,specifically to 0.13 nm RMS or better. The constitutions of exemplaryinterferometers which may be applied here are described under the fifththrough eleventh embodiments of the present invention shown in FIGS.10a-22.

At step S5, the causes of error in the wavefront aberration measured atstep S4 is broken down into an alignment error component (for eachaspheric surface) and a shape error component for each surface.

Specifically, a computer uses, for example, known optical systemautomatic correction software, assigns the position of test surface 38(spacing, inclination and shift) and the shape of test surface 38 asvariables, initializes the measurement values of the wavefrontaberration, and performs optimization so that the wavefront aberrationapproaches zero. The difference between the position and shape of testsurface 38 when optimized and the position and shape of test surface 38prior to optimization corresponds to the alignment error (positionalerror) and shape error, respectively.

At step S6, the alignment error calculated at step S5 is evaluated todetermine whether it is sufficiently small. If it is not small enough,the flow operation proceeds to step S7 where the alignment error isadjusted. If it is small enough, the flow proceeds to step S8.

At step S7, alignment of optical element 36 in optical system 37 isadjusted based on the alignment error calculated at step S5, followingwhich flow returns to step S4.

Note that the sequence of operations between steps S4 and S7 arerepeated until the alignment error calculated at step S5 is sufficientlysmall.

At step S8, the difference between the shape error (shape error isolatedby the most recent iteration of step S5) in the final wavefrontaberration (wavefront aberration as determined by the most recentiteration of step S4) and the final measured aspheric surface shape datacalculated in step S2 is calculated. This difference corresponds to thesystematic error of aspheric-surface-shape-measuring interferometer 22H.This error corresponds to the shape error of reference surface (Fizeausurface) 70 in the aspheric-surface-shape-measuring (Fizeau-type)interferometer 22H.

At step S9, the final aspheric surface shape data measured at step S2 iscorrected by the amount of the systematic error calculated at step S8,and test surface 38 is reworked using small tool grinding apparatus 400based on this corrected aspheric surface shape data. At this time,optical element 36 having test surface 38 is removed from optical system37 of which it is a part before corrective grinding operations can becarried out.

After steps S1 through S9 have been executed, optical system 37 isreassembled and the wavefront aberration is measured usinginterferometer 22J shown in FIG. 10a. The measured values are againseparated into an alignment error component and a shape error componentfor each surface, and the surface error is verified to determine whetherit is smaller than previously measured.

By numerous repetitions of the series of procedures including machiningof aspheric test surface 38, assembly in optical system 37, measuring ofwavefront aberration, and determining the systematic error inaspheric-surface-shape-measuring interferometer 22H as described above,systematic errors in aspheric-surface-shape-measuring interferometer 22Hcan be identified. Furthermore, if such errors are large (e.g., 2 nm RMSor greater), aspheric-surface-shape-measuring interferometer 22H mustitself be corrected (i.e., the surface shape of aspheric referencesurface 70 must be corrected).

If the measurement values during subsequent measurements and machiningare continuously corrected by the amount of the systematic error inaspheric-surface-shape-measuring interferometer 22H as calculated bythis procedure and this then used as data during operations using thecorrective grinding apparatus 400, an aspheric surface 38 can bemachined with good accuracy.

Since measurement accuracy, and in particular reproducibility, withaspheric-surface-shape-measuring interferometer 22H of the fourthembodiment are excellent, the above-described calibration method isextremely effective.

Furthermore, should existence of systematic errors be confirmedthereafter as a result of wavefront aberration measurement based at theexposure wavelength or other such measurements performed during aproduction run, systematic error may be corrected at each such occasionso as to constantly approach design values.

In addition, after machining the aspheric surface 38 using the machiningand measurement procedures based on the present invention, opticalsystem 37 is assembled and a reflective film (not shown) must be appliedto each surface 38 to be made reflective prior to measurement of thewavefront aberration. The shape of surface 38 may change under theinfluence of stress from the film when applying and removing (e.g., toperform corrective grinding) the reflective film. Although thereproducibility of this change should be less than 0.1 nm RMS, this isnot attainable. Nevertheless, the majority of the surface change issecond- and fourth-order components (power components and third-orderspherical aberration components), and the higher-order components aresmall. Second-order and fourth-order surface change components can becompensated to a certain degree by adjusting the surface spacing. Inother words, it is sufficient to ensure that the reproducibility of thesurface changes associated only with higher-order components are held to0.1 nm RMS or smaller. This can be accomplished by sufficient reductionof the stress from the film.

As described above, the present invention provides anaspheric-surface-shape measuring interferometer displaying goodreproducibility, and moreover makes it possible to measure wavefrontaberration with high precision. In addition, the present inventionpermits improvement in the absolute accuracy of precision surfacemeasurements in an aspheric-surface-shape-measuring interferometer. Inaddition, the present invention permits manufacture of a projectionoptical system having excellent performance.

Adoption of the present invention also makes it possible to accuratelyverify the shape of a null wavefront, as well as the transmissioncharacteristics of such a null wavefront, without the need to use areflective standard. Moreover, adoption of an interferometer systemaccording to the present invention makes it possible to calibrate anaspheric null element with high precision and in a short period of time.

Furthermore, the wavefront-aberration-measuring interferometers of thefifth through eleventh embodiments of the present invention discussedabove can be assembled as part of an exposure apparatus. In particular,when an SOR undulator of a wavelength which may be used for exposure isused as light source in a wavefront-aberration-measuringinterferometers, as was the case in the fifth and sixth embodiments,this will be favorable since the light source unit can also serve as theexposure light source. When a laser plasma X-ray source of a wavelengthwhich may be used for exposure in a wavefront-aberration-measuringinterferometers, as was the case in the seventh through tenthembodiments, this will be favorable since the light source unit can alsoserve as the exposure light source. In addition, thewavefront-aberration-measuring interferometers of the eleventhembodiment of the present invention requires a laser light source to befurnished separate from the exposure light source. However, this laserlight source can also serve as light source for an alignment system oras light source for an autofocus system in the exposure apparatus. Inaddition, in the wavefront-aberration-measuring interferometers of thefifth through eleventh embodiments of the present invention, when thislight source is shared by the exposure apparatus, detector 60 serving asdetector may also be fashioned such that it is removable from theexposure apparatus. In this case, the wavefront aberration of projectionoptical system 37 can be measured by attaching such a removable unit tothe exposure apparatus in the event that maintenance or the like isrequired. Consequently, there will be no need to provide a dedicatedwavefront-aberration-measuring interferometer for each and everyexposure apparatus, permitting reduction in the cost of the exposureapparatus.

In addition, while detector 60 has been adopted as detector in the fifththrough tenth embodiments of the present invention discussed above, amember having a function that converts emitted light in the soft X-rayregion to visible light (for example, a fluorescent plate) may beprovided at the position of the detector 60 and used in place thereof,and the visible light from this member may be detected by a detectorsuch as a CCD.

Furthermore, the embodiments of the present invention discussed abovedescribe a manufacturing method of a projection optical system 37 in thecontext of an exposure apparatus that uses soft X-rays of wavelengtharound 10 nm as exposure light, wavefront-aberration-measuringinterferometers ideally suited to the measurement of the wavefrontaberration of this projection optical system 37, surface-shape-measuringinterferometers ideally suited to measurement of the surface shape of areflective surface in this projection optical system 37, and acalibration method for such an interferometer. However, the presentinvention is not limited to this soft X-ray wavelength. For example, thepresent invention can be applied to a projection optical system orwavefront-aberration-measuring interferometer for hard X-rays ofwavelength shorter than soft X-rays, and to a surface-shape-measuringinterferometer that measures the surface shape of an optical element ofa hard X-ray projection optical system, and can also be applied to thevacuum ultraviolet region (100 to 200 nm) of wavelength longer than softX-rays. Furthermore, measurement and manufacturing of a precision muchgreater than hitherto possible becomes possible if the present inventionis applied to a vacuum-ultraviolet projection optical system orwavefront-aberration-measuring interferometer, or to surface shapemeasurement of an optical element in a vacuum-ultraviolet projectionoptical system.

Thus, the present invention is not to be limited by the specific modesfor carrying out the invention described above. In particular, while thepresent invention has been described in terms of several aspects,embodiments, modes, and so forth, the present invention is not limitedthereto. In fact, as will be apparent to one skilled in the art, thepresent invention can be applied in any number of combinations andvariations without departing from the spirit and scope of the inventionas set forth in the appended claims, and it is intended to cover allalternatives, modifications and equivalents as may be included withinthe spirit and scope of the invention as defined in the appended claims.

What is claimed is:
 1. A method of manufacturing a projection opticalsystem capable of projecting a pattern from a reticle onto aphotosensitive substrate, comprising the steps of: a. measuring a shapeof a test surface of an optical element that is a component of theprojection optical system by causing interference between light fromsaid test surface and light from an aspheric reference surface whilesaid test surface and said aspheric reference surface are heldintegrally and in close proximity to one another; b. assembling saidoptical element in the projection optical system and measuring thewavefront aberration of the projection optical system; c. determining anamount by which said shape of said test surface should be correctedbased on said measured wavefront aberration obtained in said step b; andd. correcting said shape of said test surface based on said amount bywhich said shape of said test surface should be corrected as determinedin said step c.
 2. A method according to claim 1, wherein: a. said stepc further includes the step of calculating an error in said shape ofsaid test surface as measured in said step a, based on said measuredwavefront aberration obtained in said step b; and b. said amount bywhich said shape of said test surface should be corrected is determinedfrom said calculated error, said measured wavefront aberration and saidshape of said test surface.
 3. A method according to claim 1, wherein:a. said step of calculating an error in said shape of said test surfaceincludes separating said measured wavefront aberration into an testsurface positional error component, a test surface shape errorcomponent, and a residual component when said positional error componentis substantially corrected; and b. wherein error in said surface shapeis calculated based on a component in said residual componentattributable to said shape error component and said shape of said testsurface.
 4. An interferometer calibration method for measuring a surfaceshape of an optical element of an optical system, the method comprisingthe steps of: a. interferometrically measuring the surface shape of theoptical element to obtain a surface shape measurement value; b.assembling the optical system by including the optical element in theoptical system; c. measuring a wavefront aberration of the opticalsystem; d. separating said wavefront aberration into a componentcorresponding to positional error of the surface shape and a componentcorresponding to surface shape error; e. correcting said positionalerror component and calculating said surface shape error component; andf. correcting said surface shape measurement value using said surfaceshape error component as calculated in said step e.